PET 3322 Chapter 6 Part B Notes PDF

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PET 3322 Chapter 6 Part B Notes ____________________________________________________________________________

6.5 Bone Development 

Ossification (osteogenesis) is the process of bone tissue formation  Formation of bony skeleton begins in month 2 of development  Postnatal bone growth occurs until early adulthood  Bone remodeling and repair are lifelong

Formation of the Bony Skeleton  Bone replaces fibrous membranes and hyaline cartilage of fetal skeleton after week 8  Endochondral ossification  Bone forms by replacing hyaline cartilage  Bones are called cartilage (endochondral) bones  Form most of skeleton  Intramembranous ossification  Bone develops from fibrous membrane  Bones are called membrane bones  Endochondral Ossification  Forms essentially all bones inferior to base of skull, except clavicles  Begins in month 2 of development  Begins at primary ossification center in center of shaft  Blood vessels infiltrate  Osteoblasts form  Five main steps in the process of ossification (Endochondral Ossification) 1.) Bone collar forms around diaphysis of cartilage model 2.) Central cartilage in diaphysis calcifies, then develops cavities 3.) Periosteal bud invades cavities, leading to formation of spongy bone a. Bud is made up of blood vessels, nerves, red marrow, osteogenic cells, and osteoclasts 4.) Diaphysis elongates, and medullary cavity forms a. Secondary ossification centers appear in epiphyses 5.) Epiphyses ossify a. Hyaline cartilage remains only in epiphyseal plates and articular cartilages  Intramembranous ossification: begins within fibrous connective tissue membranes formed by mesenchymal cells o Forms frontal, parietal, occipital, temporal, and clavicle bones  Four major steps are involved 1.) Ossification centers are formed when mesenchymal cells cluster and become osteoblasts

2.) Osteoid is secreted, then calcified 3.) Woven bone is formed when osteoid is laid down around blood vessels, resulting in trabeculae 4.) Lamellar bone replaces woven bone, and red marrow appears Postnatal Bone Growth  Long bones growth lengthwise by interstitial (longitudinal) growth of epiphyseal plate o Result of aging  Bone increases thickness through appositional growth o Result of circumstance  Bones stop growing during adolescence o Some facial bones continue to grow slowly through life Growth in Length of Long Bones  Interstitial growth requires presence of epiphyseal cartilage in the epiphyseal plate  Epiphyseal plate maintains constant thickness  Growth plates are in longer bones such as arms, and legs  Epiphyseal plate consists of five zones o Resting (quiescent) zone o Proliferation (growth) zone o Hypertrophic zone o Calcification zone o Ossification (osteogenic) zone 1.) Resting (quiescent) zone a. Area of cartilage on epiphyseal side of epiphyseal plate that is relatively inactive 2.) Proliferation (growth) zone a. Area of cartilage on diaphysis side of epiphyseal plate that is rapidly dividing b. New cells formed move upward, pushing epiphysis away from diaphysis, causing lengthening 3.) Hypertrophic zone a. Area with older chondrocytes close to diaphysis 4.) Calcification Zone a. Surrounding cartilage matrix calcifies; chondrocytes die and deteriorate 5.) Ossification Zone a. Closest to diaphysis b. Everything is calcifying  Near end of adolescence, Chondroblasts divide less often  Epiphyseal plate closure occurs when epiphysis and diaphysis fuse  Bone lengthening ceases o Females – occurs around 18 years of age o Males – occurs around 21 years of age Growth in Width  Growing bones widen as they lengthen through appositional growth

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Osteoblasts beneath periosteum secrete bone matrix on external bone Osteoclasts remove bone on endosteal surface

Hormonal Regulation of Bone Growth  Growth Hormone – most important hormone in stimulating epiphyseal plate activity in infancy and childhood  Thyroid Hormone – modulates activity of growth hormone, ensuring proper proportions  Testosterone (males) and estrogen (females) at puberty: promote adolescent growth spurts o End growth by inducing epiphyseal plate closure Bone Deposit  New bone matrix is deposited by osteoblasts  Osteoid seam – band of unmineralized bone matrix that marks area of new matrix  Calcification front – abrupt transition zone between osteoid seam and older mineralized bone Bone Resorption  Resorption is function of osteoclasts o Dig depressions or grooves as they break down matrix o Secrete lysosomal enzymes and protons that digest matrix  Osteoclasts also phagocytize demineralized matrix and dead osteocytes o Digested products are transcytosed across cell and released into interstitial fluid and then into blood Control of Remodeling  Remodeling occurs continuously but is regulated by genetic factors and two control loops 1.) Hormonal controls a. Negative feedback loop that controls blood calcium levels 2.) Response to mechanical stress Hormonal Controls o Parathyroid hormone (PTH): Produced by parathyroid glands in response to low blood calcium levels  Stimulate osteoclasts to resorb bone o Calcitonin – produced by parafollicular cells of thyroid gland in response to high levels of blood calcium levels  Response to mechanical stress o Bones reflect stresses they encounter o Wolfs Law states that bones grow or remodel in response to demands placed on them Control of Remodeling 

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Mechanical stress causes remodeling by producing electrical signals when bone is deformed o Compression/tension changes fluid flows within canaliculi, which may also stimulate remodeling Hormonal controls determine whether and when remodeling occurs in response to changing blood calcium levels, but mechanical stress determines where it occurs

6.7 Bone Repair  Fractures are breaks o During youth most fractures result from trauma o In old age, most results from weakness of bone due to bone thinning Fracture Classification  Three “either/or” fracture classifications o Position of bone ends after fracture  Nondisplaced – ends retain normal position  Displaced – ends are out of normal alignment o Completeness of break  Complete – broken all the way through  Incomplete – not broken all the way through o Whether skin is penetrated  Open (compound): skin is penetrated  Closed (simple): skin is not penetrated Fracture Treatment and repair  Fibrocartilaginous callus formation o Capillaries grow into hematoma o Fibroblasts secrete collagen fibers to span break and connect broken ends  Bony callus formation o Within one week, new trabeculae appear in fibrocartilaginous callus o Callus in converted to bony (hard) callus of spongy bone o Bony Callus formation continues for about 2 months  Bone Remodeling o Begins during bony callus formation and continues for several months o Compact bone is laid down to reconstruct shaft walls

6.8 Bone Disorders

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Imbalances between bone deposit and bone resorption underlie nearly every disease that affects the human skeleton 3 major bone diseases o Osteomalacia and rickets o Osteoporosis o Paget’s disease

Osteomalacia and Rickets  Osteomalacia o Bones are poorly mineralized o Happens most in adults o Osteoid is produced but calcium salts not adequately deposited o Results in soft, weak bones  Rickets (Osteomalacia of children) o Results in bowed legs and other bone deformities Osteoporosis  Osteoporosis is a group of diseases in which bone resorption exceeds deposit  Matrix remains normal, but bone mass declines  Risk factors for osteoporosis o Most often aged, postmenopausal women o Estrogen plays a role in bone density, so when levels drop at menopause, women run higher risk  Men are less prone due to protection by the effects of testosterone  Additional risk factors for osteoporosis: o Petite body form o Insufficient exercise to stress bones o Males with prostate cancer taking androgen-suppressing drugs  Treating osteoporosis o Traditional treatments  Calcium  Vitamin D supplements Paget’s Disease  Excessive and haphazard bone deposit and resorption cause bone to be made fast and poorly o Called pagetic bone

Developmental Aspects of Bone  Most long bones begin ossifying by 8 weeks, with primary ossification centers developed by week 12 Age-Related Changes in Bone  In children and adolescents, bone formation exceeds resorption  Bone density changes over lifetime are largely determine by genetics  Bone mass, mineralization and healing ability decrease with age beginning in fourth decade o Except bones of skull o Bone loss is greater in whiles and in females

Fracture Type

Description and Comments

Spiral

Bone fragments into three or more pieces.

Epiphyseal

Particularly common in the aged; whose bones are more brittle Bone is crushed Common in porous bone subjected to extreme trauma, as in a fall

Depressed

Green Stick

Comminuted

Broken bone portion is pressed inward Typical of skull fracture Bone breaks incompletely, much in the way a green twig breaks. Only one side of the shaft breaks; the other side bends. Common in children whose bones have relatively more organic matrix and are more flexible than those of adults Bone fragments into three or more pieces Particularly common in the aged whose bones are more brittle Bone is crushed. Common in porous bones subjected to extreme trauma, as in a fall.

Compression...